The demand for realizing a super-high-speed (over 40 Gbit/s, i.e., 100 Gbit/s, for example) optical transmission system has been increasing rapidly. For this reason, the development has been underway, for the practical realization of an optical transmission system adopting a multi-value modulation system (for example an RZ-DQPSK modulation system using quadrature phase modulation) that has been applied to the radio system. However, as the transmission signal speed increases, solving problems related to the viability of the electric signal circuit, and problems related to the degradation of optical transmission signals (such as the transmission signal spectrum degradation due to an optical filter, the signal degradation due to chromatic dispersion and accumulation of optical noises) becomes harder.
As a method for solving these problems to realize a large-capacity long-distance transmission system, an optical transmission system adopting the polarization division multiplexing and digital coherent detection has been attracting attention. The research and development for the commercialization of these techniques are in progress, and the techniques are disclosed in, for example, a document 1 (G. Charlet et al., “Transmission of 16.4 Tbit/s Capacity over 2,550 km using PDM QPSK Modulation Format and Coherent Receiver” presented at the OFC '08 Paper PDP3.), a document 2 (J. Renaudier, et al., “Linear Fiber Impairments Mitigation of 40-Gbit/s Polarization-Multiplexed QPSK by Digital Processing in a Coherent Receiver,” J. Lightwave Technology., vol. 26, No. 1, pp. 36-42, January 2008.), and a document 3 (O. Bertran-Pardo et al., “Nonlinearity Limitations When Mixing 40-Gb/s Coherent PDM-QPSK Channels With Preexisting 10-Gb/s NRZ Channels” IEEE Photonics Technology Letters, Vol. 20, No. 15, pp. 1314-1316, August 2008.).
According to the polarization division multiplexing, two data streams are transmitted using two polarized waves having the same wavelength and being orthogonal to each other. For this reason, the polarization division multiplexing contributes to the improvement of the characteristics of the electric signal generation circuit, cost reduction, size reduction and power-consumption reduction, as the modulation speed is reduced to half. In addition, effects due to the quality degradation factors such as the dispersion in the optical transmission path are reduced, improving the characteristics of the optical transmission system as a whole. For example, patent document 1 (Japanese Laid-open Patent Publication No. 62-024731) and patent document 2 (Japanese Laid-open Patent Publication No. 2002-344426) disclose transmission systems using the polarization division multiplexing.
In an optical signal transmitter that generates a polarization multiplexed signal, a modulator is provided for each polarization signal. For this reason, a difference in optical power between polarized waves of the optical signal may occur due to the variation of the characteristics (for example, optical loss) between the modulators, or, the variation of the optical losses of an optical splitter, an optical combiner and the like. The difference in optical power of the polarized waves causes the degradation of the transmission characteristics.